Thin film transistor and flat panel display including the same

ABSTRACT

A thin film transistor includes: a gate electrode; source and drain electrodes insulated from the gate electrode; an organic semiconductor layer that is insulated from the gate electrode and is electrically connected to the source and drain electrodes; an insulating layer that insulates the gate electrode from the source and drain electrodes or the organic semiconductor layer; and an ohmic contact layer that is interposed between the source/drain electrodes and the organic semiconductor and contains a compound having a hole transporting unit. By providing the ohmic contact layer, the ohmic contact between source/drain electrodes and the organic semiconductor layer can be effectively achieved and the adhesive force between the source/drain electrodes and the organic semiconductor layer is increased. In addition, a flat panel display having improved reliability can be obtained using the thin film transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/582,534, filed on Oct. 18, 2006, which claims the benefit of KoreanPatent Application No. 2005-99353, filed on Oct. 20, 2005, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a thin film transistor (TFT)and a flat panel display including the same, and more particularly, to aTFT in which an ohmic contact between an organic semiconductor layer andsource/drain electrodes is achieved and an adhesive force between theorganic semiconductor and the source/drain electrodes is increased, anda flat panel display including the TFT.

2. Description of the Related Art

Thin film transistors (TFTs) are used as switching devices forcontrolling pixel operations and as driving devices for operating pixelsin flat panel displays such as liquid crystalline display devices (LCD),organic light-emitting display devices, inorganic light-emitting displaydevices, and the like.

TFTs include a semiconductor layer including source/drain regions and achannel region interposed between the source region and drain region, agate electrode insulated from the semiconductor layer and located in aregion corresponding to the channel region, and source and drainelectrodes respectively contacting the source and drain regions.

Organic TFTs include an organic semiconductor layer composed of anorganic semiconductor material. Organic TFTs can be manufactured at lowtemperatures, and thus, plastic substrates can be used. Due to thisadvantage of organic TFTs, much research into organic TFTs has beenperformed recently. For example, Korean Patent Publication No.2004-0012212 discloses an organic TFT.

However, it is difficult to achieve an ohmic contact betweensource/drain electrodes and the organic semiconductor layer of anorganic TFT because of a difference in the work function between thematerials of the source/drain electrodes and the organic semiconductorlayer. In addition, since the source and drain electrodes are formed ofan inorganic material and the organic semiconductor layer is formed ofan organic material, the adhesive force between the source/drainelectrodes and the organic semiconductor layer is not strong. Therefore,improvements in these areas are desirable.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a thin film transistor (TFT) inwhich an ohmic contact between an organic semiconductor layer andsource/drain electrodes is achieved, and the adhesive force between thesource/drain electrodes and the organic semiconductor layer isincreased, and provide a flat panel display including the TFT.

According to an aspect of the present invention, there is provided athin film transistor comprising: a gate electrode; source and drainelectrodes insulated from the gate electrode; an organic semiconductorlayer that is insulated from the gate electrode and is electricallyconnected to the source and drain electrodes; an insulating layer thatinsulates the gate electrode from the source and drain electrodes or theorganic semiconductor layer; and an ohmic contact layer that isinterposed between the source and drain electrodes and the organicsemiconductor, and that comprises a material that has a work functionthat is greater than a work function of a material of the source anddrain electrodes and less than a work function of a material of theorganic semiconductor layer.

According to another aspect of the present invention, there is provideda thin film transistor including: a gate electrode; source and drainelectrodes insulated from the gate electrode; an organic semiconductorlayer that is insulated from the gate electrode and electricallyconnected to the source and drain electrodes; an insulating layer thatinsulates the gate electrode from the source and drain electrodes or theorganic semiconductor layer; and an ohmic contact layer interposedbetween the source/drain electrodes and the organic semiconductor, andthat contains a hole transporting compound and/or a compound having ahole transporting unit.

According to another aspect of the present invention, there is provideda method of forming a thin film transistor comprising: forming a gateelectrode; forming an insulating layer; forming source and drainelectrodes in predetermined regions corresponding to opposite ends ofthe gate electrode; forming an organic semiconductor layer; and formingan ohmic contact layer between the source and drain electrodes and theorganic semiconductor layer, wherein the ohmic layer comprises a holetransporting compound and/or a compound including a hole transportingunit.

According to another aspect of the present invention, there is provideda method of forming a thin film transistor comprising: forming a gateelectrode; forming an insulating layer; forming source and drainelectrodes in predetermined regions corresponding to opposite ends ofthe gate electrode; forming an organic semiconductor layer; and formingan ohmic contact layer between the source and drain electrodes and theorganic semiconductor layer, wherein the ohmic layer comprises amaterial that has a work function that is greater than a work functionof a material of the source and drain electrodes and less than a workfunction of a material of the organic semiconductor layer.

According to another aspect of the present invention, there is provideda flat panel display including the thin film transistor in respectivepixels, wherein the source or drain electrodes of the thin filmtransistor is connected to a pixel electrode.

In a TFT according to the present invention, in addition that the ohmiccontact between source/drain electrodes and the organic semiconductorlayer can be effectively achieved, the adhesive force between thesource/drain electrodes and the organic semiconductor layer isincreased. In addition, a flat panel display having improved reliabilitycan be obtained using the TFT.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIGS. 1 through 3 are sectional views illustrating structures of thinfilm transistors (TFTs) according to embodiments of the presentinvention; and

FIG. 4 is a sectional view of an organic light-emitting displayincluding a TFT according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIG. 1 is a sectional view of a thin film transistor (TFT) 10 accordingto an embodiment of the present invention. The TFT 10 includes asubstrate 11, a gate electrode 12, an insulating layer 13, source anddrain electrodes 14 a and 14 b, an ohmic contact layer 16, and anorganic semiconductor layer 15, which are sequentially stacked upon oneanother.

The substrate 11 may be a glass, plastic, or metal substrate, but is notlimited thereto. The glass substrate may be formed of silicon oxide,silicon nitride, and the like. The plastic substrate may be formed of aninsulating organic compound. For example, the insulating organiccompound may be selected from the group consisting of polyethersulfone(PES), polyacrylate (PAR), polyetherimide (PEI), polyethylenenaphthalate (PEN), polyethyleneterephthalate (PET), polyphenylenesulfide (PPS), polyallylate, polyimide, polycarbonate (PC), celluloustriacetate (CAT), and cellulose acetate propionate (CAP), but is notlimited thereto. The metal substrate may include at least one selectedfrom the group consisting of carbon, iron, chrome, manganese, nickel,titanium, molybdenum, stainless steel (SUS), an Invar alloy, a Zinconelalloy, and a Kovar alloy, but is not limited thereto. The metalsubstrate may be a metal foil. If flexibility is desired, a plastic ormetal substrate can be used for the substrate 11.

A buffer layer, a barrier layer, or an impurities diffusion inhibitionlayer may be formed on one surface or both surfaces of the substrate 11.In particular, when the substrate 11 is a metal substrate, an insulatinglayer (not shown) may be further formed on the substrate 11.

According to the embodiment of FIG. 1, the gate electrode 12 having apredetermined pattern is formed on the substrate 11. For example, thegate electrode 12 may be formed of a metal or a metal alloy, such as Au,Ag, Cu, Ni, Pt, Pd, Al, Mo, an Al:Nd alloy, an Mo:W alloy, etc. but thematerial of the gate electrode 12 is not limited thereto.

According to the embodiment of FIG. 1, the insulating layer 13 is formedon the gate electrode 12 to cover the gate electrode 12. The insulatinglayer 13 may be formed of an inorganic compound, such as a metal oxideor a metal nitride, or an organic compound, such as an insulting organicpolymer, but the material of the insulating layer 13 is not limited tothereto.

According to the embodiment of FIG. 1, the source and drain electrodes14 a and 14 b are formed on the insulating layer 13. As shown in FIG. 1,the source and drain electrodes 14 a and 14 b may overlap with a part ofthe gate electrode 12, but the structure of the source and drainelectrodes 14 a and 14 b is not limited to thereto. Non-limitingexamples of the material for the source and drain electrodes 14 a and 14b include metals, such as Au, Pd, Pt, Ni, Rh, Ru, Ir, Os, Al and Mo,metal alloys of at least two metals, an Al:Nd alloy, an MoW alloy,metallic oxides, such as indium tin oxide (ITO), indium zinc oxide(IZO), NiO, Ag₂O, In₂ 0 ₃-Ag₂O, CuAlO₂, SrCu₂ 0 ₂, and Zr-doped ZnO.Combinations of two or more of the above-mentioned metals or metallicoxides can be used.

In particular, when the ohmic contact layer 16 is formed on the sourceand drain electrodes 14 a and 14 b using a self-assembled monolayermanufacturing process, the source and drain electrodes 14 a and 14 b maybe formed of an oxidizable metal or a metallic oxide.

According to the embodiment of FIG. 1, the organic semiconductor layer15 is formed on the source and drain electrodes 14 a and 14 b. Examplesof an organic semiconductor material for the organic semiconductor layer15 include pentacene, tetracene, anthracene, naphthalene, α-6-thiophen,α-4-thiophen, perylene and a derivative thereof, rubrene and aderivative thereof, coronene and a derivative thereof, perylenetetracarboxylic diimide and a derivative thereof, perylenetetracarboxylic dianhydride and a derivative thereof, polythiophene anda derivative thereof, polyparaphenylene vinylene and a derivativethereof, polyparaphenylene and a derivative thereof, polyfluorene and aderivative thereof, polythiophene vinylene and a derivative thereof, apolythiophene-heterocyclic aromatic copolymer and a derivative thereof,an oligonaphthalene and a derivative thereof, an oligothiophene ofα-5-thiophene and a derivative thereof, a metal-containing or metal-freephthalocyanine and a derivative thereof, pyromellitic dianhydride and aderivative thereof, pyromellitic diimide and a derivative thereof, orthe like. Combinations of two or more of these materials can be used.

According to the embodiment of FIG. 1, the ohmic contact layer 16 formsan ohmic contact between the source/drain electrodes 14 a and 14 b andthe organic semiconductor layer 15. Therefore, a material for the ohmiccontact layer 16 may have a work function that is in between the workfunction of the material of the source/drain electrodes 14 a and 14 band the work function of the material of the organic semiconductor layer15. For example, when the source and drain electrodes 14 a and 14 b areformed of ITO having a work function of about 4.8-5.0 eV, and theorganic semiconductor layer 15 is formed of pentacene having a workfunction of about 5.2-5.4 eV, there is a difference in work function ofabout 0.6 eV and thus, an ohmic contact is not effectively achieved.Therefore, by forming the ohmic contact layer 15 using a material thathas a work function between the work function of the source and drainelectrodes 14 a and 14 b and the work function of the organicsemiconductor layer 15, the contact barrier between the source and drainelectrodes 14 a and 14 b and the organic semiconductor layer 15 can bereduced. Thus, an ohmic contact can be realized. In addition, becausethe ohmic contact layer 16 is located in the interface between thesource/drain electrodes 14 a and 14 b and the organic semiconductorlayer 15, the ohmic contact layer 16 increases the adhesive forcebetween the source/drain electrodes 14 a and 14 b and the organicsemiconductor layer 15.

Considering the work functions of materials which form the source/drainelectrodes 14 a and 14 b and the organic semiconductor layer 15, acompound having a hole transporting unit that is included in the ohmiccontact layer 16 may have a work function of 4.5-5.5 eV. In this case, acontact barrier reduction effect can be achieved due to the ohmiccontact layer 16.

The ohmic contact layer 16 may include a hole transporting compound.Non-limiting examples of the hole transporting compound include

and the like.

In the above formula of hole transporting compounds, R₁, R₂, R₃, R₄, R₅,R₆ and R₇ may independently be hydrogen or a substituted orunsubstituted C₆-C₃₀ aryl group, (such as, for example, a substituted orunsubstituted C₆-C₂₀ aryl group) or a substituted or unsubstitutedC₅-C₃₀ heteroaryl group (such as, for example, a substituted orunsubstituted C₅-C₂₀ hetero-aryl group). At least one of R₁, R₂ and R₃,and at least one of R₄, R₅, R₆ and R₇ should be a substituted orunsubstituted C₆-C₃₀ aryl group, or a substituted or unsubstitutedC₅-C₃₀ hetero-aryl group. R₈ may be hydrogen or a substituted orunsubstituted C₁-C₃₀ alkyl group, such as, for example, a C₁-C₂₀ alkylgroup.

In the above formula of hole transporting compounds, Ar₁ may be asubstituted or unsubstituted C₆-C₃₀ arylene group (such as, for example,a substituted or unsubstituted C₆-C₂₀ arylene group) or a substituted orunsubstituted C₅-C₃₀ heteroarylene group (such as, for example, asubstituted or unsubstituted C₅-C₂₀ heteroarylene group).

In the above formula of hole transporting compounds, n may be an integerfrom 1 to 10, such as, for example, an integer from 1 to 5.

In the above formula of hole transporting compounds, Q₁, Q₂, Q₃, Q₄ andQ₅ may be independently selected from N, C, P and S.

Non-limiting examples of the hole transporting compound includeN,N′-bis(3-methyphenyl)-N,N′-diphenyl-[1,1-biphenyl]-4,4′-diamine(TPD)-N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (NPD: including α-NPDand β-NPD), 1,3,5-tricarbazolyl benzene,4,4′-biscarbazolylbiphenyl(CBP), polyvinylcarbazole,m-biscarbazolylphenyl, 4,4′-biscarbazolyl-2,2′-dimethylbiphenyl,4,′4″-tri(N-carbazolyl)triphenylamine,1,3,5-tri(2-carbazolylphenyl)benzene,1,3,5-tris(2-carbazolyl-5-methoxyphenyl)benzene,bis(4-carbazolylphenyl)silane, BMA-Nt, TBA, o-TTA, p-TTA, m-TTA,Spiro-TPD, TPTE, and the like, but are not limited thereto. The termsTPD, α-NPD, β-NPD, CBP BMA-Nt, TBA, o-TTA, p-TTA, m-TTA, Spiro-TPD, andTPTE refer to the following compounds:

When the ohmic contact layer 16 is formed of a hole transportingcompound as described above, various known deposition methods or coatingmethods can be used.

Alternatively or additionally, the ohmic contact layer 16 may include amaterial that includes a hole transporting unit. Such a material may be,for example, a self-assembled monolayer (SAM) formed of a repeating unitthat includes the hole transporting unit. The hole transporting unit canbe the bound radical of any of the hole transporting compounds describedabove. That is, the hole transporting unit can be any of the holetransporting compounds described above, except existing as a boundmoiety instead of as a free compound. In this context, the term“radical” simply refers to a group or moiety that is bound to a largercompound or structure.

As a particular, non-limiting example, the ohmic contact layer 16according to an aspect of the present invention may be a self-assembledmonolayer(SAM) including a repeating unit of formula (1) below and/or amoiety of formula (2) below:

In formulas (1) and (2), X₁ and X₂ may each be a single bond, asubstituted or unsubstituted C₁-C₂₀ alkylene group, such as, forexample, a C₁-C₁₅ alkylene group, or —Z₁-NH-C(═O)-O-Z₂—. Z₁ and Z₂ mayeach be a substituted or unsubstituted C₁-C₂₀ alkylene group, such as,for example, a C₁-C₁₅ alkylene group.

In formula (1), Y₁ is a hole transporting unit. For example, the holetransporting unit can be the bound radical of any of the holetransporting compounds described above. In other words, the holetransporting unit can be any of the hole transporting compoundsdescribed above, except existing as a bound moiety instead of as a freecompound.

In formula (2), Y₂, W₁ and W₂ are each independently selected fromhydrogen, a halogen atom, a C₁-C₂₀ alkyl group, a C₁-C₂₀ alkoxy group,or a hole transporting unit. However, at least one of Y₂, W₁ and W₂should be a hole transporting unit. The hole transporting unit isdescribed above.

In formulas (1) and (2), * and *′ indicate binding with a surface of theTFT, in particular the surface of the source and drain electrodes 14 aand 14 b, as in the embodiment of FIG. 1 or the surface of the organicsemiconductor layer 15, as in the embodiment of FIG. 2, described below.

The SAM may include an unit of formula (3) and/or (4) below:

In formulas (3) and (4), * and *′ indicate binding with a surface of thesource and drain electrodes 14 a and 14 b or the surface of the organicsemiconductor layer 15, as in the embodiment of FIG. 2 (discussedbelow).

When the ohmic contact layer 16 according to an embodiment of thepresent invention is an SAM as described above, the ohmic contact layer16 may be formed using a self-assembled monolayer manufacturing process.The self-assembled monolayer manufacturing process may be carried out byway of various reactions, such as a hydrolysis reaction, a condensationreaction, and the like.

Throughout this specification, where an aryl group, a heteroaryl group,an arylene group or a heteroarylene group is mentioned, such as, forexample, in the definition of R₁, R₂, R₃, R₄, R₅, R₆ and R₇ or Ar₁ inthe examples of the hole transporting compound or hole transportingunit, such a group may include at least two or more carbocyclic rings,which may be attached together or may be fused.

The aryl group, heteroaryl group, arylene group or heteroarylene groupmay be each substituted by a halogen atom, a cyano group, a hydroxylgroup, a C₁-C₂₀ alkyl group, a C₆-C₃₀ aryl group, or a C₅-C₃₀ heteroarylgroup. In addition, the C₁-C₂₀ alkyl group may be substituted by ahalogen atom, a cyano group, a hydroxyl group, a C₁-C₂₀ alkyl group, aC₆-C₃₀ aryl group, or a C₅-C₃₀ heteroaryl group.

The heteroaryl group or heteroarylene group may include at least one ahetero atom selected from the group consisting of N, P and S.

The ohmic contact between the source/drain electrodes 14 a and 14 b andthe organic semiconductor layer 15 can be realized, and the adhesiveforce therebetween can be increased by the ohmic contact layer 16. As aresult, electrical properties, lifespan, etc., of the TFT 10 can beimproved.

Although the ohmic contact layer 16 is shown in FIG. 1 as being formedon the source and drain electrodes 14 a and 14 b, the structure of theohmic contact layer 16 can be varied. For example, the ohmic contactlayer 16 may extend to below a channel region 15 a of the organicsemiconductor layer 15. In particular, when the ohmic contact layer 16extends to below the channel region 15 a of the organic semiconductorlayer 15, charge trapping, etc., of the insulating layer 13 can beinhibited, and the crystalline property of the organic semiconductorlayer 15 and the adhesive force between the ohmic contact layer 16 andthe organic semiconductor layer 15 can be improved.

FIG. 2 is a sectional view of a TFT 10′ according to another embodimentof the present invention. The TFT 10′ includes a substrate 11, a gateelectrode 12, an insulating layer 13, an organic semiconductor layer 15,an ohmic contact layer 16, and source and drain electrodes 14 a and 14b, which are sequentially stacked upon one another.

For each of the layers forming the TFT 10′ of FIG. 2, the detaileddescription above relating to the composition of the correspondinglayers of the TFT 10 in FIG. 1 can be referred to. When the ohmiccontact layer 16 of the TFT 10 is a self-assembled monolayer including arepeating unit or a compound of formula 1, 2, 3 or 4 above, * and *′indicate binding with a surface of the organic semiconductor layer 15.

FIG. 3 is sectional view of a TFT 10″ according to another embodiment ofthe present invention. The TFT 10″ includes a substrate 11, source anddrain electrodes 14 a and 14 b, an ohmic contact layer 16, an organicsemiconductor layer 15, an insulating layer 13, and a gate electrode 12,which are sequentially stacked upon one another. For each of the layersforming the TCT 10″ of FIG. 3, the detailed description of thecomposition of the corresponding layers of the TFT 10 in FIG. 1 can bereferred to.

A TFT according to aspects of the present invention may be formed usingvarious methods. A method of manufacturing a TFT according to anembodiment of the present invention may include: forming an insulatinglayer to cover a gate electrode formed on a substrate; forming sourceand drain electrodes in predetermined regions of the insulating layercorresponding to both ends of the gate electrode; forming an ohmiccontact layer on the source and drain electrodes; and forming an organicsemiconductor layer to cover the ohmic contact layer.

A method of manufacturing a TFT according to another embodiment of thepresent invention may include: forming an insulating layer to cover agate electrode formed on a substrate; forming an organic semiconductorlayer on the insulating layer; forming an ohmic contact layer on theorganic semiconductor layer; and forming source and drain electrodes inpredetermined regions of the ohmic contact layer corresponding to bothends of the gate electrode.

A method of manufacturing a TFT according to another embodiment of thepresent invention may include: forming source and drain electrodes on asubstrate; forming an ohmic contact layer on the source and drainelectrodes; forming an organic semiconductor layer on the ohmic contactlayer; forming an insulating layer to cover the organic semiconductorlayer; and forming a gate electrode in predetermined regions of theinsulating layer corresponding to source and drain electrodes.

When the source and drain electrodes include an oxidizable metal, theforming of the source and drain electrodes may further include oxidizingthe surfaces of the source and drain electrodes. This operation isperformed in order to increase the adhesive force between thesource/drain electrodes and the ohmic contact layer which will be formedlater.

The oxidizing of the surfaces of the source and drain electrodes can beimplemented using various methods. For example, a method of annealingthe surfaces of the source and drain electrodes in an atmosphericcondition, such as, for example, in an oxygen atmosphere, a method oftreating the surfaces of the source and drain electrodes with a gas,such as, for example, oxygen plasma, a method of chemically treating thesurfaces of the source and drain electrodes with an oxidant, such as,for example, hydrogen peroxide, or other methods can be used. Themethods that can be used to oxidize the surfaces of the source and drainelectrodes are not limited to these examples.

The forming of the ohmic contact layer may be carried out using a knowndeposition method; a known coating method, such as, for example, spincoating, deep coating, micro contact printing, inkjet printing, etc.; ora known self-assembled monolayer manufacturing method. In theself-assembled monolayer manufacturing method, a catalyst may be furtherused to facilitate a reaction, such as hydrolysis, condensation, etc.,involved in forming the ohmic contact layer.

A TFT having the above-mentioned structure may be included in a flatpanel display, such as a liquid crystal display (LCD), an organiclight-emitting display, etc.

FIG. 4 is a sectional view of an organic light-emitting displayaccording to an embodiment of the present invention as a flat paneldisplay including a TFT described above. In particular, FIG. 4illustrates a single sub-pixel of an organic light-emitting display.Each sub-pixel of an organic light-emitting display includes an organiclight-emitting device (OLED) as a self-luminous element and at least oneTFT.

The organic light-emitting display includes various pixel-patterns, suchas, for example, red, green, and blue pixels, according to theluminescent color of the OLED.

Referring to FIG. 4, a gate electrode 22 having a predetermined patternis formed on a substrate 21, and an insulating layer 23 is formed tocover the gate electrode 22. Source and drain electrodes 24 a and 24 bare formed on the insulating layer 23. An ohmic contact layer 26 isformed on the source and drain electrodes 24 a and 24 b. The detaileddescription of the ohmic contact layer 26 can be referred to above.

An organic semiconductor layer 25 is formed on the ohmic contact layer26 to cover the ohmic contact layer 26. The organic semiconductor layer25 includes source/drain regions and a channel region connected to thesource/drain regions.

After forming the organic semiconductor layer 25, a passivation layer 27is formed to cover the TFT 20. The passivation layer 27 is formed as asingle-layered or multi-layered structure, and may be formed of anorganic material, an inorganic material, or an organic/inorganiccomposite material.

A pixel defining layer 28, which defines a pixel, is formed on thepassivation layer 27. An organic layer 32 of the OLED 30 is formed on apixel electrode 31 and extends to a part of the pixel defining layer 28.

The OLED 30 displays predetermined image information by emitting lightof a red, green or blue color according to the flow of a current. TheOLED 30 includes the pixel electrode 31 connected to one of the sourceand drain electrodes 24 a and 24 b, a facing electrode 33 covering theentire pixel, and the organic layer 32 interposed between the pixelelectrode 31 and the facing electrode 33. The present invention is notlimited to this structure, and various structures of organiclight-emitting displays can be applied.

The organic layer 32 may be a low-molecular weight organic layer or apolymer organic layer. When the organic layer 32 is a low-molecularweight organic layer, the organic layer 32 may have a structureincluding one or combinations of a hole injection layer (HIL), a holetransport layer (HTL), an emission layer (EML), an electron transportlayer (ETL), and an electron injection layer (EIL). Examples of organicmaterials for the low-molecular weight organic layer include copperphthalocyanine (CuPc), N,N-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine(NPB), tris-8-hydroxyquinoline aluminum (Alq3), etc. The small-molecularweight organic layer can be formed using, for example, vacuumdeposition.

When the organic layer 32 is a polymer organic layer, the organic layer32 may have a structure including an HTL and an EML. The HTL may beformed of poly-3,4-ethylendioxythiophene (PEDOT), and the EML may beformed of a poly-para-phenylenevinylene(PPV)-based or polyfluorene-basedpolymer material using screen printing, inkjet printing, etc.

The organic layer 32 is not limited to the above-described organiclayers, and may have various structures.

The pixel electrode 31 may act as an anode, and the facing electrode 33may act as a cathode. Alternatively, the polarities of the pixelelectrode 31 and the facing electrode 33 may be reversed.

Unlike a method of forming an organic light-emitting display, a methodof manufacturing an LCD further includes forming a lower substrate byforming a lower alignment layer (not shown) covering the pixel electrode31.

A TFT according to aspects of the present invention can be installed ineach sub-pixel as illustrated in FIG. 4, or in a driver circuit (notshown) where no image is formed.

In the organic light-emitting display, a flexible plastic substrate canbe used as the substrate 21.

Hereinafter, aspects of the present invention will be described ingreater detail with reference to the following example. The followingexample is for illustrative purposes only and is not intended to limitthe scope of the invention.

EXAMPLE

A substrate including a gate electrode formed of MoW (100-nm thick), aninsulating layer formed of SiO₂ (200-nm thick), and source and drainelectrodes formed of ITO (100-nm thick) was prepared. The substrate wassoaked in triethoxy-(triphenylamino) butyl-silane solution (50 mManhydrous toluene) for three hours. The substrate was washed withtoluene, acetone, and then isopropanol, and dried at 120° C. for an hourand cured. The substrate was coated with acetic acid as a dehydrationcondensation catalyst, and left at 50° C. for three hours. The substratewas washed with toluene and then methanol, and dried at 120° C. for anhour to form an ohmic contact layer including an SAM including a unit offormula (3). Next, an organic semiconductor layer was formed bydepositing pentacene to a thickness of 70 nm to cover the source anddrain electrodes and the ohmic electrode, thereby forming an organic TFTaccording to the present invention.

As described above, in a TFT according to aspects of the presentinvention, the ohmic contact between the source/drain electrodes and theorganic semiconductor layer can be effectively achieved, and theadhesive force between the source/drain electrodes and the organicsemiconductor layer is increased. Thus, a flat panel display withimproved reliability can be realized using the TFT.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A thin film transistor comprising: a gate electrode; source and drainelectrodes insulated from the gate electrode; an organic semiconductorlayer that is insulated from the gate electrode and is electricallyconnected to the source and drain electrodes; an insulating layer thatinsulates the gate electrode from the source and drain electrodes or theorganic semiconductor layer; and an ohmic contact layer that isinterposed between the source and drain electrodes and the organicsemiconductor, and that comprises a material that has a work functionthat is greater than a work function of a material of the source anddrain electrodes and less than a work function of a material of theorganic semiconductor layer.
 2. The thin film transistor of claim 1,wherein the ohmic contact layer is formed using a deposition method, acoating method, or a self-assembled monolayer manufacturing process. 3.The thin film transistor of claim 1, wherein the ohmic contact layerprovides an adhesive force between the source and drain electrodes andthe organic semiconductor layer.
 4. The thin film transistor of claim 1,wherein the source and drain electrodes include at least one selectedfrom the group of Au, Pd, Pt, Ni, Rh, Ru, Ir, Os, Al and Mo, an Al:Ndalloy, an MoW alloy, ITO, IZO, NiO, Ag₂O, In₂ 0 ₃-Ag₂O, CuAlO₂, SrCu₂ 0₂ and Zr-doped ZnO.
 5. Thin film transistor of claim 1, wherein theorganic semiconductor layer includes at least one of pentacene,tetracene, anthracene, naphthalene, α-6-thiophene, α-4-thiophene,perylene and a derivative thereof, rubrene and a derivative thereof,coronene and a derivative thereof, perylene tetracarboxylic diimide anda derivative thereof, perylene tetracarboxylic dianhydride and aderivative thereof, polythiophene and a derivative thereof,polyparaphenylenevinylene and a derivative thereof, polyparaphenyleneand a derivative thereof, polyfluorene and a derivative thereof,polythiophenevinylene and a derivative thereof,polythiophene-heterocyclic aromatic copolymer and a derivative thereof,an oligonaphthalene and a derivative thereof, an oligothiophene ofα-5-thiophene and a derivative thereof, metal-containing or metal-freephthalocyanine and a derivatives thereof, pyromellitic dianhydride and aderivative thereof, and pyromellitic diimide and a derivative thereof.6. A flat panel display comprising the thin film transistor of claim 2in each pixel, wherein the source or drain electrode of the thin filmtransistor is connected to a pixel electrode.